Obstructive Sleep Apnea one of the most common disorders in the U.S. Lower oxygen levels associated with Obstructive Sleep Apnea (OSA) is now known to be a major cause of cardiovascular morbidity including heart attack and stroke. At present expensive polysomnography is used to identify these patients but not on a sufficient scale to provide diagnosis as a practical matter. The development of a diagnostic system which can allow simplified diagnosis of obstructive sleep apnea by the primary care physician would be a major step. The prevention of hundreds of thousands of annual excess deaths, stroke and heart attacks associated with obstructive sleep apnea through simplified recognition of this disorder is the most important purpose of the present invention. These excess deaths are occurring annually in a great part due to the lack of availability of this technology resulting in a vast pool of undiagnosed cases of Sleep Apnea and other breathing disorders. Despite the fact that obstructive sleep apnea is easily treated, both the patient and the family are often completely unaware of the presence of this dangerous disease, thinking the patient just a “heavy snorer”.
Obstructive sleep apnea often develops insidiously as a patient enters middle age and begins to snore. The major cause is an increase in fat deposition (often age related) in the neck which results in narrowing of the airway. (In fact the probability that a 40 year Id has sleep apnea is directly related to his or her neck circumference). When the muscle tone of the upper airway diminishes during sleep coupled with negative pressure associated with inspiration through this somewhat narrow airway results in collapse of the upper airway in a manner analogous to the collapse of a cellophane straw. This results in airway obstruction and, effectively chokes off all air movement The choking patient (still asleep) begins to struggle and inhales more forcibly, thereby, further lowering upper airway pressure and causing further collapse of the upper airway. During this time, substantially no air movement into the chest occurs and the patient experiences a progressive fall in oxygen (similar to the fall occurring early in drowning). The fall in oxygen produces central nervous system stimulation contributing to hypertension and potential heart and blood vessel injury and finally results in arousal. Upon arousal, increase in airway muscle tone opens the airway and the patient rapidly inhales and ventilates quickly to correct the low oxygen levels. Generally, the arousal is brief and the patient is not aware of the arousal (or of the choking since this occurs during sleep). Once oxygen levels have been restored, the patient begins again to sleep more deeply, upper airway tone again diminishes, the upper airway collapses and the cycle is repeated stressing the heart with low oxygen in a repetitive fashion. Often this repeating cycle over many years eventually results in damage to the heart muscle and/or the coronary arteries. As the patient ages, the consequences of undiagnosed obstructive sleep apnea is often either a progressive decline in heart muscle function (and eventual heart failure) or heart infarction.
The duration and severity of each apnea event is quite variable from patient to patient and with the same patient throughout the night. Indeed, the disease process represents a spectrum of severity from mild snoring, which is associated with incomplete and inconsequential airway obstruction, to severe apneas which can result in fatal hypoxemia.
This disease commonly results in excessive daytime sleepiness and can disrupt cognitive function during the day due to fragmentation of sleep during the night associated with recurrent arousals of which the patient is not aware.
Although this disease commonly affects obese patients, it may occur in patients with any body habitus. Because this disease is so common and because it presents with the subtle and common symptoms of excessive daytime sleepiness, morning headache, and decreasing ability to concentrate during the day, it is critical that an inexpensive technique for accurately diagnosing and treating this disease be developed. Traditionally, this disease has been diagnosed utilizing a complex and expensive multi-channel polysomnogram. This is generally performed in a sleep lab and involves the continuous and simultaneous measurement and recording of an encephalogram, electromyogram, electroculogram, chest wall plethysmogram, electrocardiogram, measurements of nasal and oral air flow, and pulse oximetry. These, and often other, channels are measured simultaneously throughout the night and these complex recordings are then analyzed to determine the presence or absence of sleep apnea.
The problem with this traditional approach is that such complex sleep testing is expensive and limited to laboratories. Since sleep apnea is so common, the cost of diagnosing obstructive sleep apnea in every patient having this disease in the United States is prohibitive. It is critical that a new, inexpensive technique of accurately diagnosing sleep apnea be developed.
For example, nocturnal oximetry alone has been used as a screening tool to screen patients with symptoms suggestive of sleep apnea to identify whether or not oxygen desaturations of hemoglobin occur. Microprocessors have been used to summarize nocturnal oximetry recordings and to calculate the percentage of time spent below certain values of oxygen saturation However, oxygen desaturation of hemoglobin can be caused by artifact, hypoventilation, ventilation perfusion mismatching. For these reasons, such desaturations identified on nocturnal oximetry are not specific for sleep apnea and the diagnosis of sleep apnea has generally required expensive formal polysomnography.
The diagnosis of sleep disorders often involves polysomnography (PSG), the monitoring and recording over an extended period of time of the temporal variations in the amplitude of the patient's sleep-impacted, physiological parameters, including: heart rate, eye blink activity, airflow rate, thorax and abdomen respiration rates, the blood's oxygen saturation level, electroencephalograms (EEG), electrooculograms (EOG), and electromyograms (EMG). Such intensive monitoring activities are typically conducted in clinical settings by trained PSG technicians who utilize expensive monitoring equipment having multiple sensors that are tethered to a centralized recording system and power supply.
For several decades, the recordings of such physiological parameters were provided by strip chart recorders that produced long strips of paper with ink markings that displayed the varying physiological parameters. The clinician would then examine such records and “score” each abnormal sleep event that occurred. This practice continues today with the clinician now viewing computer screens displaying the varying physiological parameters.
More recently, a number of portable recording systems for screening and diagnosing sleep disorders have been marketed. These systems range from multi-channel, PSG-style systems to much simpler units that monitor only one or more of the possible physiological parameters of interest. However, these multi-channel, portable systems remain technically complex, expensive and usually require trained PSG technicians to supervise their use.
Some of the newer of these portable systems offer comprehensive software for display and analysis of the collected sleep data, and some offer automatic sleep event scoring. However, such scoring has been found to have varying degrees of reliability due to the technical problems associated with assuring good signal fidelity in the monitored parameters. Thus, all of these systems recommend for accurate identification of abnormal sleep events that the data be interpreted and evaluated by experienced clinicians or trained PSG technicians.
Since PSG scoring is largely subjective, experienced scorers can generally interpret with good accuracy the action and interactions of poorly shaped and time skewed signals. Although these distortions are commonly accepted as normal for manual scoring, such poor fidelity signals would be unsatisfactory for automated or computer-based scoring.
All of the current, portable sleep testing systems share common, less-than-desirable features for home use: (1) they are bedside portable, but their size and weight does not allow the patient to be ambulatory, which can be essential for diagnosing patients problems such as excessive sleepiness, (2) they are not designed for unattended use-a technician must come to the home for set-up, disconnection and data retrieval, (3) patients must be outfitted with an array of tethered electrode wires and sensors for connection to bulky body monitors or table-top consoles, and (4) most require subjective analysis of the data by highly trained, sleep professionals.
Recognizing the need for an improved apparatus or method for diagnosing of the various medical conditions of a fully ambulatory subject who exhibits temporal variations in various physiological parameters as a result of this medical condition, it is therefore a general object of the present invention to provide a novel method and ambulatory, distributed recorders system to meet such needs.